1. Field of the Invention
The present invention relates to a thermally-assisted recording head for assisting magnetic recording with heat caused by a laser beam, and a magnetic recording system using the thermally-assisted recording head.
2. Description of the Related Art
A higher recording density is demanded of a magnetic disk device mounted in a computer, etc. as one of the information recording devices, in order to store a large amount of information without enlarging the device. In order to achieve a higher recording density in a magnetic disk device, a recording medium with high coercivity is used because of the necessity to stabilize minute recording bits. To record a recording medium with high coercivity, it is necessary to concentrate high magnetic field strength on a micro region. However, this is difficult technically.
As a technology for addressing the aforementioned problem, a hybrid recording technology combining an optical recording technology and a magnetic recording technology has been considered to be one effective way. The coercivity of the medium is decreased by heating the medium with a laser beam up to the vicinity of the Curie temperature (about several hundreds of degrees centigrade) of the medium (ferromagnetic) concurrently with an applied magnetic field when recording. As a result, recording becomes easier on a recording medium with high coercivity where recording has been difficult with a related art magnetic head because of insufficient recording magnetic field strength. A magnetoresistance effect used for a related art magnetic recording is used for reading. This hybrid recording method is called thermally-assisted magnetic recording or optically-assisted magnetic recording.
In this thermally-assisted magnetic recording method, a laser beam for heating the medium is guided to the recording head. A semiconductor laser diode (hereinafter referred to as LD) which is small in size and low in power consumption is used for a laser source because of the necessity to use it in a magnetic disk device package. In the thermally-assisted magnetic recording, it is necessary to efficiently guide a beam generated by a semiconductor laser to the tip of the head, that is, to decrease propagation loss. Examples of the structure for fulfilling this demand include the following. Japanese Published Unexamined Patent Application No. 2008-59645 discloses a head that is mounted with a horizontal cavity vertical emitting LD with a reflector monolithically integrated, and in which an optical waveguide and a near-field generator are integrated inside a slider. Japanese Published Unexamined Patent Application No. 2008-59691 discloses a head that is mounted with an edge emitting short cavity LD, and in which, in the same manner as the foregoing, an optical waveguide and a near-field generator are integrated inside a slider. Japanese Published Unexamined Patent Application 2010-49735 discloses a head in which an optical waveguide is integrated inside a slider.
In optical device integrated heads for thermally-assisted magnetic recording as disclosed in the above-described Japanese Published Unexamined Patent Application Nos. 2008-59645, 2008-59691, and 2010-49735, laser beams emitted from a semiconductor laser pass through the optical waveguide provided in the slider to be guided to an air bearing surface (ABS: Air Bearing Surface) of the slider. When the beams from the LD enter an upper surface of the slider to be coupled to the optical waveguide, an optical coupling loss is generated, and there are inevitably some beams which do not become a propagating mode. These beams, which do not become the propagating mode, normally become a radiation mode, and disappear somewhere. However, in a case where there are internally plural structures, such as the slider of the magnetic head, which are nontransparent to the laser beam, or there is a reflecting surface such as a trailing edge of the head, light in the radiation mode might be reflected by such structures or reflecting surface and stray around in the slider without disappearing. This light is referred to as stray light. Coupling of the stray light to the optical waveguide is likely to cause interference with light in the propagating mode. The interference between light beams different in phase causes instability of the light mode, resulting in spatial and temporal fluctuation in the intensity and the mode spot. This instability of the mode causes instability in the supply of laser beams to the near-field generator provided in the vicinity of the ABS, thereby leading to instability in the near-field light to be generated and instability in the amount of heat required for recording onto a recording medium. As a result, an expected thermal assisting cannot be achieved, and magnetic recording becomes impossible.
Therefore, preventing generation of such stray light or eliminating stray light is an important issue in stable thermally-assisted recording.
Hereinafter, generation of stray light will be described by using a specific structure of the magnetic head. In the thermally-assisted magnetic recording heads disclosed in the Japanese Published Unexamined Patent Application Nos. 2008-59645 and 2008-59691, in order to reduce optical loss, optical components are mutually integrated, thereby minimizing the number of coupling points requiring optical coupling. Also, a light path from a light source to a recording medium is shortened so as to reduce optical loss.
However, even if the number of optical coupling portions is reduced, the coupling loss of waveguide light is caused when light passes through the coupling points between components, thereby causing attenuation of the intensity of the waveguide light. The semiconductor LD is generally made of a compound semiconductor with GaAs, GaN, or InP as a substrate material. On the other hand, the slider of the magnetic head is made of Al—Ti—C and Al2O3 as a sintered body of Al2O3 and TiC, and magnetic metal. Therefore, the semiconductor LD and the slider cannot be the same component, and these are separate components. Consequently, a coupling portion inevitably exists between the semiconductor LD and the slider, and hence the optical loss is caused when light passes through the coupling portion. Lost light does not propagate through the optical waveguide of the slider. Some of produced loss light is reflected by an upper surface of the slider, that is, a light incidence plane to return toward the semiconductor LD. The rest of the loss light enters the upper surface of the slider, however, does not propagate through the optical waveguide. Commonly, light not propagating through the optical waveguide is radiated outside the slider other than through the optical waveguide or absorbed by a structure provided in the slider, and disappears. However, some of the light strays around in the slider and becomes stray light. The stray light is reflected by a structure in the slider or a surface of the slider and might be indirectly coupled to the optical waveguide. The optical coupling of stray light to the optical waveguide provided in the slider causes optical interference with the waveguide light directly coupled to the optical waveguide and propagating therethrough from the semiconductor LD. Since there is a difference in light path between these light beams, their phases are different. Further, the interference state spatially and temporally varies depending on different conditions such as temperature. When interference occurs, a designed waveguide light mode becomes difficult to forecast. Further, the mode temporally fluctuates, causing instability in the mode of the light to be supplied to the near-field generator.
Such instability in the light to be supplied causes instability in the near-field light to be generated by the near-field generator, thereby preventing the amount of heat required for thermally-assisted recording from being controlled and transmitted to a recording medium. As a result, magnetic recording state instability is caused. Therefore, stray light needs to be eliminated in order to ensure the stable supply of the amount of heat to the recording medium. As described above, the stray light causes not only the optical loss, but also the interference with the propagating-mode light propagating through the optical waveguide, resulting in instability in its original propagating mode because the stray light is different in phase from the propagating light mode. The stray light is a known phenomenon in optical disk drives, waveguide devices for optical networking or the like. In these fields, since light is not allowed to propagate through a structure, such as a slider, internally including a minute structure, there are not many cases where the light lost without being coupled to the optical waveguide becomes stray light. Also, even if stray light occurs, it seems to be easy to identify a reflector that causes the stray light because of their relatively simple structures, and take measures to prevent generation of the stray light.
However, in the thermally-assisted magnetic recording heads, as disclosed in Japanese Published Unexamined Patent Application Nos. 2008-59691 and 2010-49735, the slider is a structure having a size of about several hundreds of microns, with an optical waveguide added to a related art head structure. The optical waveguide is disposed at a position close to a trailing edge of the slider, several tens of microns therefrom. Moreover, a recording magnetic pole, a coil for magnetizing the magnetic pole, a read sensor, a floating height control heater, wiring for supplying power thereto and the like are complexly disposed in the vicinity of the optical waveguide, thereby creating a situation in which light non-coupled to the optical waveguide is reflected and likely to become stray light.
FIG. 1 is an exemplary sectional view of the vicinity of a thermally-assisted magnetic recording head of a magnetic disk device according to the related art.
A horizontal cavity vertical emitting semiconductor LD100, in which a reflecting mirror 104 formed with a 45-degree tapered surface is monolithically integrated, is mounted over a slider 110. A laser beam emitted from the semiconductor LD100 becomes a waveguide light 204, and the waveguide light 204 passes through an optical waveguide 111 provided in the slider 110 to a magnetic recording medium (a magnetic disk) 120. The light path is indicated by the arrows.
In the horizontal cavity vertical emitting semiconductor LD100, a laser beam produced by an active layer 101 is reflected by the reflecting mirror 104. A reflected laser beam 201 reaches perpendicular to the surface of the horizontal cavity vertical emitting semiconductor LD100 on the side on which the slider 110 is located, and exits therefrom.
The laser beam 201 exiting from the surface of the LD100 on the side on which the slider 110 is located reaches an upper surface of the slider 110 and enters the optical waveguide 111 provided in a manner penetrating the slider 110 in a thickness direction of the slider 110. A thin antireflection film 119 is provided on an upper surface (the upper surface of the optical waveguide 111) of the slider 110, thereby preventing optical loss due to reflection and preventing reflected return light to the LD100. In this case, in addition to the loss due to reflection, coupling loss occurs if there is a difference in the natural mode-field shape of the light between the LD100 and the optical waveguide 111 that is provided in a manner penetrating the slider 110 in a thickness direction of the slider 110. Also, even if there is no difference in the natural mode-field shape therebetween, coupling loss occurs if there is a space between the light exit surface of the LD100 and the upper surface of the slider 110.
Actually, it is impossible to equalize the light mode-field sizes between the LD100 and the optical waveguide 111 since respective materials and structures making up the LD100 and the optical waveguide 111 are different. Also, it is almost impossible in terms of mounting technology to couple the LD100 and the optical waveguide 111 without space therebetween.
Therefore, an optical loss inevitably occurs in an optical coupling point between the LD100 and the optical waveguide 111. Light entering the slider 110 and non-coupled to the optical waveguide 111 preferably disappears, for example after passing through the slider 110 to the outside, or being absorbed by metal in the slider 110. However, this light is reflected by various structures existing in the slider 110 or a trailing edge surface of the slider 110 and likely to become stray light. The stray light might be unexpectedly coupled to the optical waveguide 111. Interference between the waveguide light 204 propagating through the optical waveguide 111 and the stray light causes irregularities in the field of the waveguide light 204, leading to instability in the supply of light. This can result in deterioration of the stability behavior of a thermally-assisted mechanism. Consequently, a mechanism for preventing generation of stray light, or eliminating or reducing generated stray light is necessary.
Furthermore, the optical waveguide 111 may have the function of converting an incident laser beam into a small spot size in the slider 110. Light with a spot size reduced in the optical waveguide 111 is projected on a near-field generator 115. Typically, the spot diameter of the light generated by the LD100 is in a range of about 2 to 4 μm, and the size of the near-field generator 115 is in a range of about 100 to 300 nm. Therefore, the light spot needs to be reduced to about one fifth to one tenth the size during propagation through the optical waveguide 111. Otherwise, sufficient light is prevented from being supplied to the near-field generator 115, leading to significant reduction in the light conversion efficiency. In view of this, a spot size converting waveguide is used to rapidly reduce the size of the light spot while propagating through the slider 110. The spot size converting waveguide is disclosed, for example in Japanese Published Unexamined Patent Application No. 2007-257753. The spot size converting waveguide rapidly reduces the light field spot size. Typically, such rapid reduction in light field spot size causes optical loss, and the spot size converting waveguide is therefore designed to minimize the optical loss.
However, since the loss caused upon reduction of the light field spot size cannot be completely eliminated, the optical loss occurs. The lost light is radiated outside the optical waveguide 111, and thus likely to become stray light in the slider 110. Therefore, the stray light generated by the spot size converting waveguide also needs to be eliminated.
FIG. 2 is an exemplary sectional view of the vicinity of a thermally-assisted magnetic recording head with a horizontal cavity edge emitting semiconductor LD 10 as a light source, unlike FIG. 1. Even when a popularized horizontal cavity edge emitting semiconductor LD is used as a light source, the phenomenon caused by optical coupling due to mounting of the slider 110 and the light source is exactly the same as FIG. 1.
As described above, on the light path from the LD to the recording medium 120, there are two major points at which stray light might occur.
When optical coupling or light spot size conversion is performed, it is impossible to completely eliminate light radiation caused by optical loss. Therefore, how to suppress the generation of stray light has become an important issue. Consequently, it is necessary to eliminate or reduce possibly becoming stray light at each stray light generation point. Accordingly, an object of the present invention is to realize a thermally-assisted magnetic recording head including a slider with a mechanism for suppressing generation of stray light, in which stability of a thermally-assist mechanism is realized and the recording performance of a magnetic disk device is improved.